The present invention relates to image pickup apparatus having a function for obtaining a wide dynamic range, synthesized image by synthesizing image signals corresponding to a plurality of frames of different exposure amounts outputted from an image pickup device.
Solid-state image pickup devices such as CCD image pickup device are generally used in image pickup apparatus as TV camera, video camera, digital camera, etc. There is a problem however that the dynamic range of a solid-state image pickup device is much narrower than that of a silver halide photographic film.
To eliminate this problem, there have been proposed techniques in which image signals corresponding to two frames taken at different exposure amounts are read out from a single image pickup device and are synthesized to obtain an image having increased dynamic range. For example, an image pickup apparatus having the following construction is disclosed in Japanese Patent Publication No. 2522015. In particular, a disclosure has been made with respect to an image pickup apparatus including: image pickup means for converting object image into electrical signals; image pickup control means for selectively executing a first mode or second mode, said first mode for successively outputting images of different exposure amounts in cycles by cyclically changing charge accumulation time at the image pickup means, image portions at suitable level being synthesized to form a synthesized frame of said predetermined cycle by respectively comparing signal level of each portion of a plurality of frames of different exposure amounts outputted from said image pickup means with a predetermined reference value, said second mode for providing output by using a constant charge accumulation time at said image pickup means; and switch control means for switching the image pickup control means to the first mode when an occurrence of object having a large luminance difference within a frame is detected in said second mode by comparing signals obtained from said image pickup means in said second mode with a predetermined level. This image pickup apparatus, when in the first mode, is capable of obtaining an image of which all the portions of a frame are at suitable signal level. Even when object having a large luminance difference exists within a frame as in a backlighted condition, lack of detail at high level portion thereof for example does not occur within the frame and it is in effect possible to provide a wider dynamic range. Further, without an operator's decision on the conditions of object, etc., it is automatically switched to the first mode by the switch control means when object having a large luminance difference exists within a frame in the second mode, making it possible to automatically correct a backlighted condition, etc.
In the above described image pickup apparatus as disclosed in Japanese Patent Publication No. 2522015, however, switching to the first mode for generating wide dynamic range, synthesized image is automatically made when an existence of object having a large luminance difference is detected within a frame which is obtained in the second mode. No consideration has been made therein about a normal image taking mode where the generation of wide dynamic range, synthesized image is automatically prohibited or about a mode for forcing the generation of wide dynamic range, synthesized image. This results in a problem of a reduced operability. Further, there is another problem in the image pickup apparatus as disclosed in the abovementioned publication that, since means for displaying ON/OFF status of the generation processing of wide dynamic range, synthesized image are not provided, it is difficult to identify the switched state of ON/OFF of synthesized image.